The items described above are generally known. In particular, every conventional rolling operation takes place in the way described above, control units for rolling stands are software-programmed and every conventional rolling device is formed in the way described above.
In the production of metal strip, in particular hot strip, there can be the problem that the tail of the strip breaks out laterally in a rolling mill. Therefore, there can be the problem that the actually desired, central path of the strip is not ensured, and unproblematic operation of the rolling device in terms of the rolling operation is not ensured.
The lateral breaking out of the strip in a horizontal direction may be caused by various physical dependences. Examples of such dependences are an unsymmetrical tensile stress profile over the width of the strip, a wedge-shaped strip cross section, a skewed position of the work rolls, an unsymmetrical form of the work rolls, etc.
In order to avoid the lateral breaking out of the strip and the concomitant disadvantages, it is known in the prior art to lower the tension in the strip on the inlet side of the rolling stands to zero. The lowering of the tension may take place, for example, by lowering a loop lifter, which is arranged between the rolling stand and a further rolling stand arranged upstream. Alternatively, the roll gap of the upstream rolling stand may also be fully or partially opened. This procedure has the disadvantage that it has a direct influence on the rolling operation as such. In particular, reducing the tension leads to stronger rolling of the strip in the rolling stand. Opening the upstream rolling stand even has the consequence of entirely or partly precluding the rolling operation that can actually be brought about in this upstream rolling stand.
A further measure taken in the prior art is to arrange segmented tension measuring rollers, that is to say loop lifters by means of which the tensile stress over the width of the strip can be sensed, ahead of or behind the rolling stand. The sensed tensile stresses can in this case serve as a basis for a closed-loop control, which counteracts the lateral breaking out of the strip. However, segmented tension measuring rollers are very expensive. Furthermore, the effectiveness of this measure has not been empirically substantiated.
JP 11 267 728 A discloses a rolling method of the type mentioned at the beginning in which it is monitored whether the tail of the strip reaches a changeover point lying ahead of the rolling stand, as seen in the rolling direction, and, as from the point in time at which the tail of the strip reaches the changeover point (changeover time), the arrangements of rolls are subjected by means of a setting device to a bending force which spreads the arrangements of rolls apart and is as great as a balancing force of the upper arrangement of rolls. The balancing force of the upper arrangement of rolls is the gravitational force that has to be compensated to keep the upper arrangement of rolls in balance, that is to say to prevent the upper arrangement of rolls from sinking onto the lower arrangement of rolls.
JP 07 144 211 A discloses a rolling method in which the operating mode of the rolling device is changed over at a point in time at which the tail of the strip passes a measuring arrangement which is arranged between the rolling stand and a holding-up element for the strip situated upstream of the rolling stand, as seen in the rolling direction.